1. Field of the Invention
This invention relates to electrosensitive compositions and recording materials and process and apparatus for using same. More particularly, the present invention relates to an electrosensitive recording sheet which is highly stable, requires only low recording voltages for recording information, and is not dependent on the ambient humidity for its operation. The present invention further relates to such electrosensitive recording materials which record information through formation of color centers by injection into the electrosensitive coating of electrons.
2. Discussion of the Prior Art
Previously-known electrosensitive papers generally fall into one of four major categories. These include the following:
(1) Electrosensitive paper based on the selective removal of a white coating on a black background through the action of an electrical spark produced by a stylus moving on the surface of the recording sheet. TELEDELTOS paper (a trademark of Western Union) is representative of this type of paper. The major inconveniences of such a recording system are the high voltage required to produce the spark (several hundred volts), the low resolution due to the uncertainty of positioning the local spark; the generation of radio frequency disturbances by the spark; and the creation of odors, fumes and powdered residues due to the burning of the white coating. Other disadvantages include the limitation of the response time to the time required to recharge the capacitor to produce the spark. The recording papers in this category are relatively insensitive to variations of humidity in the ambient atmosphere.
(2) Electrochemical paper based on electroreduction or oxidation of an organic compound, such as catechol, resulting in a change in color. The problems with such recording systems are its dependence on humidity for its operation and the necessity for keeping the recording paper in sealed drums or otherwise in a humid atmosphere prior to use. Because of the humidity dependence, if the paper stays on the recorder for extended times, its usefulness will be substantially, if not completely, impaired. Other drawbacks with this category of electrosensitive recording papers are the high consumption of the electrodes and the relatively high recording power needed (20 mA and 20 volts). Papers sold by ALDEN and/or HOGAN are representative of the class of electro-chemical papers.
(3) Electrosensitive papers which are a modification of the type (1) in which reducible pigments are dispersed in a white layer. The pigments turn dark (black) due to the combination of current and heat generated by a stylus. While such recording papers, such as sold by MOTOROLA and/or 3M eliminate some of the problems associated with the Teledeltos paper, such problems are not completely eliminated. While the size of the spark can be diminished and can be more readily localized, and thus the resolution improved, the said electrosensitive papers become more sensitive to the ambient humidity, although still generally less than with the electro-chemical papers described above. These papers also exhibit a superior response time to the conventional Teledeltos paper and produce less odor, fumes and powder residues.
(4) Electrothermosensitive papers in which a thermosensitive dye is contained in a resistive layer. In this category of paper the temperature generated by the passage of current through the resistive layer causes the thermosensitive dye to turn black to produce the trace. The major drawbacks of this paper are its slow response time and limited storage time under high temperature conditions. Representative of the electrothermosensitive papers are those types of papers produced by the JUJO COMPANY.
In addition to these four major categories there have been a number of patents which have recently issued on modifications of the electrochemical type of papers discussed above. Representative of these are U.S. Pat. Nos. 3,974,041 and 4,012,292. In these electrochemical papers the necessary humidity is released by the stylus current from zeolithic water associated in pigments present in the electrosensitive layer. As far as the present inventors are aware there have been no commerical or industrial applications of these papers.
U.S. Pat. No. 3,355,290 to Jean J. Robillard, one of the present inventors, discloses a photosensitive recording paper for electro-catalytic photography. According to this patent a dry photographic process and apparatus utilizes a sensitized medium to make a permanent visible indication of received radiation such as visible light or other radiation wherein the recording medium comprises a semiconductive oxide in which a change in state of oxidation is obtained by the selective introduction of minute amounts of a catalyst in the semiconductor oxide lattice. The introduction of the catalyst ions into the semiconductor provides a stable, high contrast, recording trace. The source of the catalytic ions is provided in a separate layer from the semiconductor layer. A photoconductor is used to modulate the flow of the catalytic ions to the semiconductor layer. As disclosed in this patent the mechanism of the image forming reaction was thought to be due to the catalytic reduction of the semiconductor oxide into the colored species. It has since been recognized by the present inventors that while this explanation still holds on a macroscopic level, the exact nature of the color forming entities was color centers formed by trapping of electrons by surface defects on the semiconductor oxide lattice produced by the combination of defects in the semiconductor oxide and absorbed catalyst ion in the vicinity of the defect. Since one catalyst ion acts on a large number of defects in the oxide, a large number of color centers can be generated by the introduction of a single catalyst ion, trapped electrons injected by the electrode used in the process and apparatus of the patent, together with the catalyst ions, form the color centers. The semiconductive oxides suggested for use in this patent include, among others, TiO.sub.2, SnO.sub.2, Sb.sub.2 O.sub.3, Bi.sub.2 O.sub.3, etc.
U.S. Pat. No. 3,490,941, also issued to Jean J. Robillard, discloses an electrosensitive paper which does not require a separate electroconductive backing layer. The mechanism of the color forming reaction is substantially the same as for the previous patent but is limited to those semiconductor oxides which can be precipitated in the paper base, such as Sb.sub.2 O.sub.3 and Bi.sub.2 O.sub.3.
U.S. Pat. No. 4,091,375 of Mr. Robillard describes a solid state flat screen passive display and memory device which has some similarities to the recording systems described in the above mentioned U.S. Pat. Nos. 3,355,290 and 3,490,941, except that the color formed can be erased and that the structure of the surface defects in the semiconductor used is such that no introduction of catalyst is needed to form the trapping centers and the injection of electrons alone is sufficient to produce the color centers. The flat screen solid state display device of this patent includes a transparent support on which are placed, in succession, a tunnel junction electron injector (including metallic layer, insulating barrier layer and thin metallic layer composite), an image screen (layer of chromophoric material which can generate color centers) and a system or array of metallic electrodes spread over the surface of the image screen.
The recording papers of U.S. Pat. Nos. 3,355,290 and 3,490,941 are designed to provide permanent records whereas the flat screen display and memory device of U.S. Pat. No. 4,091,375 is designed to provide erasable images.